e.g. FLUIDly Limitless User Interface Device,
where the acronym itself is fluid except that it pertains specifically
to User Interface.

logo used in the 2005 ACM Multimedia paper:
(FL_UI_D logo exactly as it was published in the 2005 publication[2])
but for accessibility to the visually impaired, we often
use a text-based version of logo for
FLUID, written as:
"FL_UI_D", in order to keep the
emphasis on fluidity of the
User Interfaces in particular
(as in the original graphical logo).

FL_UI_D describes a number of devices
invented by S. Mann, for adaptability to a wide range of users
regardless of skill or physical ability.

FL_UI_DI

FLUIDI, like MIDI, can be used to control
a wide variety of multimedia devices,
but it is an "undigital" (fluidly continuous) control protocol
that works for multimedia (audio, video, etc.) as well as
other devices such as fluid process control instrumentation,
sensors, and effectors, unlike MIDI which is limited by
its need to start and stop events at discrete points in time.

References

Two examples of FL_UI_D: EyeTap and Hydraulophone:
(PDF)
(LEFTMOST) Two FL_UI_D technologies (EyeTap and Hydraulophone) depicted
on a recent magazine cover; (RIGHTMOST)
Make your public park accessible using FLUIDI.
Concert based on Hydraulophone.